Caesar wants Marcus to find Decimus and kill him because he killed his father and made his mother one of his slaves. Nobody in Rome wants Marcus to succeed so his only supporters are his friends and Caesar.
Marcus and his group are on their way to find Decimus and are attacked by Procrustes and his troops. They are trying to stop him from getting to Decimus because they were sent by him and they know what Marcus and his friends are up to.
Festus wanted to run from Procrustes army so they can survive and complete their mission but the rest of the group disagreed with Festus and felt they shouldn't run and they should fight Procrustes. Marcus wants to remind Decimus what is going to happen to him when they find him because they are not going to stop tell they do.
Marcus has gotten past Procrustes guards and have gotten to him. Marcus attacks Procrustes and stabs him which ends up killing him.
Marcus and his friends travel to Decimus's estate and are charged for murder after they killed Procrustes a few days earlier and Decimus is trying to use it against them.
You're next Decimus
You murdered Procrustes!
Marcus and his friends attack Decimus's estate and kill all of his guards so they can get to him. They then kill him and go back home to tell Caesar that the man who killed his father and made his mother a slave is now gone.